Ground-based coherent backscatter radar systems are extensively used to investigate small-scale dynamics in the earth's ionosphere and related geophysical process(es) in the magnetosphere. At high-latitudes, HF radars are used due to the requisite orthogonal condition with the earth's magnetic field lines. Because of the effect of ionospheric refraction on the ray paths, the exact path of the radar signal through the ionosphere is then unknown. In practice, it is important to locate the radar echo sources given the echo parameters, such as group path, elevation angle, and azimuth angle. Furthermore, radar observations consist of direct backscatter from the ground. An uncertainty arises due to the difficulty in separating true ground backscatter from ionospheric scatter which fulfils the radar criteria based on the measured Doppler velocity and spectral width.;These problems are investigated in this research using a three-dimensional ray tracing computer programme, Jones3D (Jones and Stephenson, 1975). Some problems in the Jones3D code have been identified and corrected whilst modifications to the code have been made to suit the purpose of this research work. All modelling work presented in this thesis is based on two HF radars, the Halley HF radar in Antarctica and the CUTLASS HF radar in Finland. For the best comparison with radar observations, realistic ionospheric conditions are used. In the case study for the Halley HF radar in Antarctica, it is found that the radar's main propagation mode is one-hop propagation, and that the radar scatter is mostly organised in ranges in the order of E- region scatter, F- region scatter, and ground scatter. The range-bin statistical analysis suggests that the radar criteria based on the measured Doppler velocity and special width are not sufficient, and that the measured range (group path) parameter is important and should be used in separating radar ionospheric echoes from ground backscatter.